Development of a Deep Image Retrieval Network Using Hierarchical and Multi-scale Spatial Features.

Image retrieval aims to find similar images to a given query by matching features extracted directly from the images of a database. Deep convolutional neural networks provide an excellent framework for obtaining highly representative feature vectors from images to improve an image retrieval method. Deep residual networks perform better than existing deep networks, as they can incorporate useful information into the feature vectors through residual learning by designing appropriate operations in the employed residual block. One type of such information is spatial information obtained at different scales and levels of abstraction. In this paper, a novel residual block is proposed to generate a rich set of features for the task of image retrieval. The development of the residual block consists of three modules: a hierarchical spatial feature extraction module focusing on spatial information at different abstraction levels, a multi-scale feature extraction module that generates features at three different scales, and a feature fusion module. The results of experiments on various datasets and an ablation study show that the proposed residual block noticeably improves the representational capacity of the network, which, in turn, significantly enhances the retrieval performance of the deep image retrieval network.